Optically pumped, solid-state lasers have long been known but continue to offer superior performance in many applications. Such lasers include an optically active medium, typically in the shape of a cylindrical rod, having multiple electronic energy levels. A high-energy optical source pumps the active medium of the rod so as to invert the population densities of the energy levels. The lasing radiation has energy defined by the differences between the energies of the inverted levels. Two mirrors define an optical cavity at least partially extending along the axis of the laser rod so that the lasing radiation grows and resonates within the cavity. The lasing medium then lases at a somewhat lower energy (longer wavelength) than that of the pump light. One of the mirrors is made partially transmissive so as to extract a fraction of the lasing light.
A semiconductor laser diode provides an efficient source of optical pumping light. It can very efficiently convert 30 to 45% of the electrical energy input to the diode to optical energy emitted as laser light from the diode. For very high intensity lasers, semiconductor laser diodes are available in linear arrays, referred to as diode bars, which are capable of providing very high intensity radiation. These bars are primarily composed of GaAlAs and emit light in the 800 nm region of the spectrum. As illustrated in plan view in FIG. 1, such a light diode bar 10 includes a large number of parallel laser stripes 12 formed on a common GaAs substrate 14. The large number of stripes 12 are arranged in smaller groups 16 with gaps 18 separating the groups 16. Each group 16 laterally extends over about 200 .mu.m, and the entire light diode bar 10 laterally extends over about 1 cm and emits from a line 20 extending over nearly the same distance. The generally wedge-shaped light emission pattern from such a light diode bar 10 is illustrated isometrically in FIG. 2. In the vertical direction, the emission is diffraction limited from the approximately 1-.mu.m high stripe and has a full vertical angle of 60.degree.. On the other hand, the emissions from the groups 16 of laser stripes 12 have a full lateral angle of 10.degree. extending outwardly from the relatively wide line 20 and they soon overlap, but this divergent beam still needs focusing in the far field if the source is widely separated from the point of use.
The use of bar diodes as sources of pump light has introduced a set of conflicts in coupling the pump light into the laser rod, particularly if two important engineering characteristics are to be achieved: highly efficient coupling between the pump and laser light; and the mode purity of the laser beam.
The coupling between the pump and laser light involves a process in which the coupling is proportional to the overlap between the radiation field of the pump light and the radiation field of the lasing light, itself resulting from the pump light. If the radiation field of the pump light, for whatever reason, does not coincide with the radiation field of the lasing light, the non-coinciding portion of the pump light is lost and the efficiency of the solid-state laser is decreased. Hence, if the desired laser beam is round, it is preferred that the pump light present a round distribution which matches the size of the laser beam.
A yet more fundamental problem arises from the need to stabilize the lasing modes of the solid-state laser. A large-scale solid-state laser tends to support, absent counter measures, a large number of modes, which for this discussion will be restricted to transverse modes. The fundamental mode in a circularly symmetric system is the TEM.sub.00 mode, itself having two degenerate orthogonally polarized modes. This mode manifests a circular energy symmetry and a Gaussian energy distribution about the center of the rod. Higher-order modes, such as TEM.sub.01, TEM.sub.10, TEM.sub.11, TEM.sub.12, etc. have an azimuthal energy distribution introducing nodes into the optical distribution around the cross section of the rod. It is generally desired to restrict the lasing mode to the fundamental TEM.sub.00 mode with only one linear polarization, excluding all of the others. This leads to the highest performance, diffraction-limited lasers.
An efficient laser design shows a preference for TEM.sub.00 operation and has a pump light profile which has a substantial overlap with this mode.
At least two problems arise if higher-order modes are not suppressed. Higher-order modes have larger modal cross sections than the fundamental mode. If the output beam is to be focused to a spot, the higher-order radiation presents a larger virtual image of the laser source. Also, if the optical conversion efficiency in the laser rod is generally high, any pump light coupled into the higher-order modes lowers the amount of pump light available for coupling into the desired fundamental mode. That is, coupling into higher-order modes reduces the conversion efficiency into the fundamental mode.
There are basically two distinct ways of coupling pump light into a laser cavity, end pumping and side pumping.
In end pumping, as illustrated generally in FIG. 3, the diode bar 10 irradiates an optical system 26 that focuses the sheet-like or wedge-like bar beam 28 through an entrance mirror 30 so as to present a circularly symmetric beam onto a laser rod 32. An exit mirror 34 together with the entrance mirror 36 form an optical cavity about the laser rod 40. The exit mirror 34 is itself partially transmissive so that an output laser beam 42 is extracted from the cavity. This end-pumping configuration offers a number of advantages. Assuming that the beam of the pump light is circularly symmetric, the pump light both well overlaps the fundamental TEM.sub.00 mode and also by its symmetry prefers the TEM.sub.00 mode over other transverse modes. This configuration tends to be efficient if the length of the rod 40 is longer than the absorption length of the pump light, and if the pump light does not diverge substantially over the absorption length.
However, the end-pumped configuration has problems when a diode bar provides the pump light. The diode bar 10, as was described with respect to FIG. 2, inherently provides an optical source having vastly asymmetric characteristics across the two symmetric dimensions of the circular rod 40. Extra measures must be taken to convert the linearly extending output of the diode bar to the circularly symmetric pump favored for TEM.sub.00 laser output.
Measures can be taken to create an asymmetric pump mode to match the asymmetry of the diode bar. For example, Wallace et al. disclose in U.S. Pat. No. 5,103,457 an end-pumped laser system having pumping beam-shaping elements, including Brewster angle prisms, disposed at the ends of the laser rod to convert the astigmatic pump light into a generally symmetric pump field in the active medium. This effectively matches the circular laser mode to the asymmetrically shaped pump radiation. In any case, the optics required to create this mode-matching introduce undesired complexities despite the obvious advantages of the end-pumping geometry.
Additionally, the end-pumping geometry suffers from an inability to conveniently scale to high power levels. Since the mode matching must occur with optical beams that are typically only 0.5 mm in diameter, it becomes very difficult to concentrate large amounts of power into such a small spot. Normally, only 15 to 20 W of pump power can be so tightly focused. This high intensity of pump light also incurs severe materials limitations, as the heat load causes strong distortions in the laser medium. Common laser materials, such as Nd:YAG, Nd:YLF, and Nd:YVO.sub.4 all reach their operational limits in this power range. Their operation is limited by fracture, aberrated thermal lensing, stress birefringence, and other strain-optical effects.
The alternative geometry is side-pumping in which, as illustrated in the isometric view of FIG. 4, the line 20 of the diode bar 10 is aligned with an axis 43 of the laser rod 40, and the optical output of the diode bar irradiates the longitudinally extending side of the laser rod 40. This geometry has several advantages in matching the linearly extending bar diode 10 with the linearly extending laser rod 40. There is no need to focus the optical output of the diode bar 10 in the direction along the longitudinal axis 42 of the laser rod 40. Even focusing circumferentially of the laser rod may not be required if the diode bar 10 can be brought close enough to the laser rod 40 so that its beam 44 irradiates only a small portion of the circumference of the rod. As a result, the pump entrance optics may be considerably simplified. The number of diodes may be increased to thereby increase the total laser output since the length of the laser rod is not fundamentally constrained. Additionally, thermal limitations of the laser material are not encountered in the 5 to &gt;100 W output range because the pump light is distributed over a greater amount of laser crystal than in the case of end-pumping.
However, the side-pumped configuration has fundamental geometric disadvantages, especially for a cylindrically shaped laser rod. For highly efficient coupling, the bar diode 10 is positioned relatively closely to the laser rod 40 with the result that a generally wedge-shaped beam 44 irradiates a central plane of the laser rod 40. Generally, it is desired to produce a circularly shaped laser beam since other shapes typically having higher-order components that are difficult to optimize with normal optics. Also, the simple structure of FIG. 4 requires a trade off between increasing coupling efficiency by making the absorption length of the pump light shorter than the rod diameter and increasing pumping uniformity by making the absorption length longer.
The uniformity problem can be significantly reduced by coating the portions of the cylindrical sidewall of the laser rod, not including where the pump beam 44 initially strikes, with a highly reflective coating, as has been described by Ajer et al. in "Efficient diode-laser side-pumped TEM.sub.00 -mode Nd:YAG laser," Optics Letters, vol. 27, no. 24, 1992, pp. 1785-1787. If diode bar 10 is brought close to the laser rod 40, the incident aperture not covered by the reflective coating is relatively small. Thereby, the pump light is substantially trapped within the laser rod 40, with the subsequent passes across the rod tending to circularize the pump field within the rod. Ajer et al. further increase the circularity of the pump field by positioning two such laser bars 10 along the laser rod but oriented circumferentially at positions offset by 90.degree. with respect to the rod center. They reported that their structure provided a coupling efficiency of 27% in pulsed operation and a circular, TEM.sub.00 laser beam. However, their structure did not accommodate any heat sinking of the laser rod and is thus subject to deleterious thermal effects if CW operation were to be attempted.
One of the present inventors, Kmetec, has suggested in U.S. patent application Ser. No. 08/268,781, filed Jun. 30, 1994, herein incorporated by reference in its entirety, a particularly advantageous configuration for a side-pumped solid-state laser. Other art is available for many of the features of Kmetec, including U.S. Pat. No. 5,572,541 to Suni. As illustrated in the axial cross-sectional view of FIG. 5 and in the isometric view of FIG. 6, the laser rod 40 is embedded in a cylindrical cavity in a solid block 50 of copper or other highly thermally conductive material, and the rod 40 tightly contacts the copper block 50 to provide good thermal conductance. A surrounding surface 52 of the cylindrical cavity is coated with gold or other highly reflective material so as to reflect light with high efficiency. A linear slit 54 is formed between the cylindrical cavity and the exterior of the copper block 50 and is sized to closely accommodate the 60.degree. vertical emission angle from the stripe 20 of the diode bar 10 with perhaps a few reflections within the slit 54. The diameter of the rod 40, that is, its dimension along the pump incidence direction, is preferably a small fraction, e.g. less than 1/2, of the absorption length of the pump light within the rod 40, which low absorption is in direct contrast to the conventional practice for side-pumped lasers. As a result, light emitted from the diode bar 10 through the slit 54 traverses the laser rod 40 many times as it is multiply reflected by the sides 59 of the slit 54 of the gold-coated copper block 50. The cylindrical cavity of the copper block 50 serves as an optical trap so that all the incident light, minus any reflectivity losses at the gold coating at the surrounding surface 52 and any back reflections through the slit 54, is absorbed by the laser rod 40.
Even with the nearly optimal optical absorption, the lasing medium in the rod is not totally efficient, and a large fraction of the absorbed optical pump power is dissipated as heat. The configuration of FIGS. 5 and 6, however, allows effective thermal coupling by thermal conduction between the laser rod 40 and the copper block 50, which, besides having a large thermal mass, is mechanically and thermally fixed on a mount 55 that is heat sunk to cooling means, examples of which are chilled water, refrigeration systems, thermoelectric coolers, or finned radiators. Such efficient thermal conduction is especially important for laser rods generating 1 W or more of laser power.
Nonetheless, the side-pumping configuration presents some fundamental problems. The intrinsic problem is that the pump field is necessarily non-uniform across the circular rod. The reflectance from the walls of the circular cavity is somewhat less than unity; a 5% loss per reflection is considered good. If the pump-to-active coupling efficiency is to be kept high by keeping the absorption per pass substantially above the wall loss, then the resultant pump field tends to be distinctly asymmetric. As illustrated in the axial cross section in FIG. 7, the laser bar 10 irradiates the laser rod 40 with a beam 53 propagating along a central optical axis 56 assumed to intersect the center of the rod 40. The figure includes a number of items related to the invention to be described later. The beam 53 has a full Gaussian angular width in the width direction of the slit 54 of about 60.degree. in free space, but the width is substantially less inside the high-index laser rod 40, about 32.degree. full width for YAG, with the strongest portions significantly nearer the optical axis 56. The first few passes of rays of the 60.degree. divergent beam 54 through the rod 40 are schematically illustrated in FIG. 7. The effective pump field can be approximated by a cross-sectional area 58 in the general form of an ellipse. The major axis of the ellipse extends along the optical axis 56 of the beam 53 from the laser bar 10 and may be large enough to approximately equal the diameter of the rod 40; however, the minor axis of the ellipse, transverse to the major axis, is substantially less than the rod diameter. The elliptical shape of an effective pump field is only an approximation to the real non-uniform field, which undoubtedly has higher order terms. But, based on the experimental results discussed below, the elliptical field appears to be a reasonable approximation.
Kmetec attempts to circularize the pump field within the laser rod 40 by enhancing the scattering of the pump light on reflection so as to more completely randomize the pump light, but this never completely circularizes the field and further introduces additional constraints on the design. Kmetec further attempts to remove the flattened pump field by using multiple diode bars aligned at different azimuthal angles about the laser rod, for example, three bars separated by 120.degree. between any pair. While this approach removes some of the non-circular components, it can leave nodes in the pump pattern. Furthermore, the geometrically determined multiplicity of laser bars constrains the design, and the different angles introduces a large, complex mechanical structure.
Fujino discloses a design similar to the above described features of Kmetec et al. in Japanese Laid-Open Patent Application 5-335662. Golla et al. disclose in "300-W diode-laser side-pumped Nd:YAG rod laser," Optics Letters, vol. 20, no. 10, 1995 an extension of the azimuthal arrangement of multiple diode bars. They position eighteen diode laser arrays in a 9-fold symmetry, and a separate reflector is positioned on the opposite side of the laser rod from each diode laser array. This arrangement produces a pump field that was highly concentrated around the axis of the laser rod, even more highly than that of Fujino. The rod is heat sunk in a flow tube, and thermal effects become significant above about 400 W of pump power. Walker et al. disclose a somewhat similar side-pumped laser in "Efficient continuous-wave TEM.sub.00 operation of a transversely diode-pumped Nd:YAG laser," Optics Letters, vol. 19, no. 14, 1994, pp. 1055-1057. They use reflective optics to focus the pump light at the central axis of the pump rod, thus producing a highly concentrated pump field.
The elliptical pump field 58 is obviously not well matched to a circularly symmetric laser field and hence the radiation coupling efficiency is less than ideal.
Hobbs et al. in U.S. Pat. No. 5,590,147 disclose a different approach of matching the laser beam to the pump field. They arrange two arrays of laser diodes on opposed sides of a flattened laser rod and focus the light into the rod, to thus create a sheet-shaped pump field in the rod. They then use optics at the two ends of the rod to pass the laser beam multiple times through the pump field but at positions arranged along the pump irradiation direction. Thereby, a round laser beam is better matched along its entire length to the pump field.
Zhang in U.S. Pat. No. 5,515,394 teaches the importance of beam shaping both in side-pump and end-pumped lasers. In all his embodiments, he uses a slab-shaped laser body (rectangular body having sides of substantially different sizes), and uses optics between the diode bars and the slab to produce a sheet-like pump field in the laser slab. In his end-pumped laser, prisms at one end of the laser slab shapes the laser beam to better conform to the sheet-shaped pump field. Thus, Zhang's use of laser beam shaping is predicated upon his flattened rectangular laser body, and the highly collimating optics disposed between the pump source and the laser body.
A further problem with the slotted heat sink of Kmetec is the limited access of the diode bar radiation through the slit and into the laser rod. Kmetec teaches allowing the diode beam 53 to multiply reflect off the walls 59 of the slit 54 before it reaches the laser rod 40. Suni in U.S. Pat. No. 5,572,541 shows a similar light-guiding slit within a heat sink. Generally, in these structures the slit depth should be relatively short so that the 60.degree. vertically divergent radiation reaches the laser rod with only a few reflections from the slit walls. Kmetec recommends coating the slit walls 59 with gold or other reflective materials, but smoothly forming and coating a narrow slit is difficult and the overall limitation on slit length remains. A slit of relatively shallow depth means that the heat-sinking medium surrounding the rod 40 is distinctly asymmetric near the slit 54, thus introducing thermal gradients around the circumference of the rod 40. On the other hand, for better optical coupling, the number of reflections from the slit walls 59 can be reduced by increasing the width of the slit 54 or by decreasing its depth, but an increased slit width or a decreased slit depth only exacerbates the thermal and optical asymmetries associated with the slit. Overall, the optimal slit for thermal uniformity is small in width and large in depth while the optimal slit for optical coupling the slit is large in width and small deep in depth. Obviously, the two considerations are incompatible in the prior art.